It is known to prepare hot-melt adhesive compositions from polystyrene-polyisoprene-polystyrene and polystyrene-polybutadiene-polystyrene block copolymers. There are numerous uses for hot-melt adhesives, but whereas one hot-melt adhesive may be useful in a given application, it may be unsuitable in another. For example, one particular hot-melt adhesive might be excellent for bonding together paper, or cardboard, but will not form acceptable bonds between certain rubbers, or metals. Moreover, some uses require bonding between substrates formed of different materials, and a given hot-melt adhesive may bond one type of substrate but not the other. Furthermore, it may be necessary that the hot-melt adhesive be applied at relatively low temperatures to avoid damage to one or the other of the substrates, or the bond must be flexible and provide adhesive elongation. Certain products, notably disposable articles, for example multi-line products, e.g., disposable diapers, sanitary napkins, bed pads, etc., are exemplary of articles of manufacture which require the use of hot-melt adhesive having such characteristics. Other articles of manufacture requiring the use of hot-melt adhesives include such operations as magazine and book binding, and elastic gluing operations generally.
In the construction of disposable diapers, sanitary napkins, and bed pads, e.g., hot-melt adhesives capable of bonding a non-woven moisture absorbent fabric, or moisture absorbent fluffy material, usually in layers, is applied as a continuous or discontinuous film to a substrate to bond said non-woven fabric or fluffy material thereto. In the construction of multi-line products via present techniques, e.g., in the construction of disposable diapers, a non-woven fabric, or absorbent fluffy material is generally bonded to an inner ply of a soft surfaced material, or tissue, which contacts the wearer's skin. An outer ply of moisture impervious material is adhered thereto via the use of a hot-melt adhesive to minimize moisture, or liquid strike-through. Whereas the adhesive can be sprayed, brushed or otherwise applied upon the surface of a substrate as a continuous or discontinuous film, it has become the practice to apply the adhesive as fine lines across the face of a substrate, or as a multi-dot pattern having a large number of adhesive droplets. As a result, these types of construction are generally termed "multi-line" constructions, and the products as "multi-line" products. The hot-melt adhesives used in the production of multi-line products must possess certain adhesive qualities not commonly shared by all adhesives.
In general, e.g., disposable diapers are constituted of a fluid impervious polyethylene, or polypropylene outer sheet and an inner moisture absorbent sheet covered by an inner lining of non-woven tissue. Hot-melt adhesives are applied in the form of fine parallel longitudinal strips to bond the layers together, and hence must possess sufficient adhesive and cohesive strength to provide high bond strength values so that when subjected to stress the constructions cannot be easily separated. It is also necessary that the adhesives withstand high mixing and application temperatures without thermal degradation and loss of adhesive properties, as well as good heat and oxidation resistance on aging. Good adhesive performance is required at elevated temperatures since the disposable articles are worn at body temperature, and also exposed to elevated temperatures during warehousing and shipping. The hot-melt adhesive must have low adhesive viscosity so that it can be applied at low temperature in order to avoid distortion of the polyethylene or polypropylene substrates to which it is applied. Quite obviously, all hot-melt adhesives do not possess these, and other useful properties to the same degree, or even to as high degree as would be desired; even those presently commercially used for multi-line constructions. There thus remains a need for improved hot-melt adhesive compositions useful in the assembly of multi-line constructions, and disposable articles of multi-line construction formed from improved hot-melt adhesive compositions. There likewise exists a need for hot-melt adhesives useful in magazine and book binding, and elastic gluing operations generally.